Radio frequency (RF) and RFID readers are used in a variety of contexts to detect the presence of items with RFID tags or transponders attached to the items. Examples include tracking assets in a warehouse and detecting the presence of vehicles driving down a road for the purposes of toll collection. In many instances, multiple RFID readers are used to be able to detect the presence of these items in a larger physical area. However, antennas from the multiple readers can generate overlapping fields when transmitting an interrogation signal, thereby creating interference between the multiple readers.
A variety of approaches have been taken to address the issue of signal interference. U.S. Pat. No. 7,772,977 to Tiernay describes an approach of using frequency-selective filters in RFID tag to remove interference terms in a signal. United States Patent Publication No. 2012/0280796 to Gravelle et al. describes interleaving time slots for antenna transmission in a configuration where a single antenna is attached to each RFID reader, and each RFID reader is assigned to transmit in a time slot during which no RFID readers adjacent to it are transmitting. U.S. Pat. No. 8,314,687 to Gravelle et al. describes using time division multiplexing to synchronize communication of multiple RFID readers using multiple protocols. None of these approaches allows RFID readers to operate in a free-running mode, as opposed to a time division multiplexing mode, without requiring modifications to RFID tags, which implementation can be costly when implemented on a wide scale.